The present invention relates generally to sanitary absorbent articles and in particular to feminine sanitary absorbent napkins which are thin, yet highly absorbent.
Externally worn, sanitary absorbent napkins are one of the many kinds of feminine protection devices currently available. The development of materials having a high liquid absorption capacity per unit volume has allowed the required overall thickness of sanitary napkins to be reduced, thereby providing a product which is more comfortable and less obtrusive to wear. Thin sanitary napkins are generally constructed of multiple layers of material each having a particular function, as for example disclosed in U.S. Pat. No. 5,575,786 to T. W. Osborne III. The sanitary napkin disclosed in this document includes a top sheet, an acquisition or transfer sheet, an absorbent core and a barrier sheet. The top sheet serves as the initial layer onto which liquid is first deposited and comprises a material which is pervious but non-absorbent to liquids, to provide a contact surface with the wearer which remains dry. The acquisition sheet which lies between the top sheet and the absorbent core acts to spread liquid from a localized position on the top sheet over a wider area so that liquid is presented to the absorbent core over a relatively large proportion of its surface area. Thus, the acquisition layer is made from a material which has good lateral wicking characteristics. The acquisition layer also serves as an intermediate buffer, providing initial absorption and temporary retention of liquid to allow time for the liquid to be drawn into the absorbent core after its initial deposition on the top sheet.
The absorbent core serves as the main reservoir for liquid deposited on the sanitary napkin and therefore has a high liquid absorption capacity. Materials used for the absorbent core include wood pulp, creped cellulose wadding, absorbent foams and sponges, polymeric fibers and polymeric gelling agents. The material should also be capable of retaining liquid under pressure to prevent rewetting of the acquisition layer and top sheet.
The barrier sheet is made from a material which is impervious to liquid absorbed into the absorbent core and serves as a protective barrier between the absorbent core material and the wearers clothing.
In the above construction, the acquisition or transfer layer is designed to promote lateral irrigation of liquid so that the absorbent core is effective in rapidly and efficiently drawing liquid away from the acquisition layer. However, a drawback of this known construction is that liquid can be drawn all the way to the edge of the acquisition layer and wet the top sheet resulting in undesirable leakage of liquid from the napkin, discomfort to the wearer and staining of the wearer""s garments.
Therefore, there is a need for a thin sanitary absorbent napkin which reduces the risk of leakage.
According to the present invention there is provided a sanitary napkin adapted to be worn in the crotch portion of an undergarment comprising: a first liquid-permeable sheet of material, a second liquid-permeable sheet of material disposed adjacent the first sheet, an absorbent member for absorbing liquid and disposed adjacent the second sheet, said second sheet being arranged to receive liquid deposited on said first sheet and to transfer the liquid to said absorbent member, wherein said napkin has a thickness of less than or equal to about 5 mm, when dry, and a penetration time of less than 15 seconds.
The xe2x80x9cpenetration timexe2x80x9d is defined as the time taken for the napkin to absorb a predetermined quantity of a specific liquid in accordance with the test procedure described in detail below. The inventors have found that a construction which gives a penetration time of less than 15 seconds prevents dispersion of liquid to the edges of the transfer layer thereby virtually eliminating the risk of leakage through the edge of the top sheet. In contrast, it has been found that known, thin sanitary absorbent napkins, which have a tendency to leak through the above mechanism, have penetration times of about 25 seconds.
Preferably, the first liquid permeable sheet has an open pore structure and little absorption capacity to allow liquid to be drawn rapidly away from the upper, body-facing surface into the adjacent second liquid-permeable sheet.
In a preferred embodiment, the second liquid-permeable sheet comprises a material having a structure with relatively open pores for efficiently absorbing liquid from the first sheet. The second sheet may comprise a material having a density in the range from about 0.04 to 0.05 g/cc, a basis weight of between about 80 and 110 g/m2 and a thickness in the range of about 2 to 3 mm.
In a preferred embodiment, the napkin has at least one and preferably a plurality of spaced-apart, elongate channel formations arranged to direct liquid therealong for subsequent absorption into the second sheet. The channel(s) extend generally in the plane of the napkin, i.e. laterally, e.g. generally parallel to the surface of the first and/or second sheets. The channels may be formed in the first and/or second sheets and/or between the two. The channel(s) may extend obliquely to the longitudinal axis and may be linear or arcuate. The inventors have found that the provision of channels has an effect on reducing the penetration time as defined by the test procedure described hereinbelow. Advantageously, the channel(s) may be formed by applying pressure to localized regions of the napkin, for example by embossing, which has the simultaneous effect of densifying the material at the floor of the channel, enabling liquid to flow further along the channel before being absorbed. Advantageously, the second sheet may include fusible fibers such as thermoplastic fibers. It has been found that thermoplastic fibers conveniently assist in providing the requisite combination of liquid permeability and wicking properties and at the same time assist in the formation of the channels by embossing and in preserving the shape of the channels over time.
In a preferred embodiment, the absorbent member includes superabsorbent material and may include a mixture of cellulosic fibers and superabsorbent material.
Preferably, the absorbent member comprises an absorbent material having a basis weight of from about 100 g/m2 to about 700 g/m2 which has been air-laid as a bottom layer of pulp, a middle layer of pulp and superabsorbent polymer disposed in amongst the pulp, and a top layer containing at least some pulp.
In a preferred embodiment, the absorbent member has a density of more than about 0.25 g/cc, and more preferably from about 0.3 to 0.4 g/cc.
Preferably, the absorbent member includes from about 5 weight percent to about 60 weight percent superabsorbent polymer, and more preferably in the range of about 30 to 40 weight percent superabsorbent polymer.
In a preferred embodiment, the absorbent material has a basis weight in the range from about 150 g/m2 to about 350 g/m2 and more preferably in the range from about 200 g/m2 to about 300 g/m2.